Astronauts Christina Koch and Jessica Meir conducted the 421st spacewalk beginning at 8 a.m. ET Friday in a mission to make an urgent repair to its power system and replace a batter charge/discharge unit (BCDU) that failed earlier this month after new lithium-ion batteries were installed on the station's exterior, NASA said. The BCDU regulate the amount of power the station's batteries collect from the massive solar arrays as the station orbits the nighttime side of Earth. The failure has not affected daily operations on the ISS, but the repair was pushed up three days for it to occur on Friday. In total, there are 24 BCDUs installed on the station.

Koch, an electrical engineer, and Meir, a marine biologist, are expected to take around five hours to replace the battery unit on the exterior of the station. Friday will be is Meir's first spacewalk and Koch's fourth time outside the ISS.

A press release from NASA announcing the spacewalk noted that this was Koch and Meir's first spaceflight, who were selected as astronaut candidates back in 2013 as part of the first class that contained 50% of women.

"Meir will be the 15th woman to spacewalk, and the 14th U.S. woman. It will be the 43rd spacewalk to include a woman. Women have been performing spacewalks since 1984, when Russian cosmonaut Svetlana Savitskaya spacewalked in July and NASA astronaut Kathryn Sullivan conducted a spacewalked in October," the release added.

NASA had originally planned an all-female spacewalk back in March, but had to call it off when they realized there weren't enough spacesuits of the right size available on the station, prompting criticism of the space agency for not having enough suits on hand for two women to participate in a spacewalk together.